Emissive, fixed format especially direct view displays such as Light Emitting Diode (LED), Field-Emission (FED), Plasma, EL and OLED displays have been used in situations where conventional CRT displays are too bulky and/or heavy and provide an alternative to non-emissive displays such as Liquid Crystal displays (LCD). Fixed format means that the displays comprise an array of light emitting cells or pixel structures that are individually addressable rather than using a scanning electron beam as in a CRT. Fixed format relates to pixelation of the display as well as to the fact that individual parts of the image signal are assigned to specific pixels in the display. Even in a color CRT, the phosphor triads of the screen do not represent pixels; there is neither a requirement nor a mechanism provided, to ensure that the samples in the image in any way align with these. The term “fixed format” is not related to whether the display is extendable, e.g. via tiling, to larger arrays. Fixed format displays may include assemblies of pixel arrays, e.g. they may be tiled displays and may comprise modules made up of tiled arrays which are themselves tiled into supermodules. Thus “fixed format” does not relate to the fixed size of the array but to the fact that the display has a set of addressable pixels in an array or in groups of arrays. Making very large fixed format displays as single units manufactured on a single substrate is difficult. To solve this problem, several display units or “tiles” may be located adjacent to each other to form a larger display, i.e. multiple display element arrays are physically arranged side-by-side so that they can be viewed as a single image. Transferring image data by packetized data transmission to the various display devices makes segregation of the displayed image into tiles relatively easy. At the junction of the tiles, usually some means to hide the join is applied. Such could be an opaque mask, as is for instance done in the case of tiled LCD displays, where the image of individual LCD panels is projected on a black matrix. To maintain a uniform appearance to the display, this mask is extended over the complete surface of the display and comprises an array of openings that coincide with the light emitting pixel structures of the display, or an array of openings that coincides with a group of light emitting pixel structures of the display (e.g. array of 4×4 pixels in one opening of the mask). OLED displays provide certain advantages for tiled displays such as light-weight, ease of manufacture, wide angle of view, and the ability to use back-connectors which allows close tiling with the smallest joint between tiles.
The human eye can detect very subtle color shifts or brightness changes and the optical non-uniformity and asymmetry introduced at the borders of tiles can produce disturbing optical effects. The traditional method of manufacture of OLED displays results in a pixel structure as shown schematically in FIG. 1 with a transparent substrate 2, usually a glass substrate, being closest to the viewer and facing in the display direction. Behind this substrate a series of layers 4-8 are deposited, e.g. at least a first transparent electrode 4, the organic light emitting element 6 and a second electrode 8. An organic LED material is deposited for each color in each pixel structure, e.g. three color elements 6, red, green and blue, for each pixel structure. Thus, each pixel structure can emit white light or any color by controlling the light energy emitted from each color element of a pixel structure. Usually additional layers are deposited such as electron and hole transport layers 7, 5 (see FIG. 1 that has been adapted from FIG. 4-13 of “Display Interfaces”, R. L. Myers, Wiley, 2002). Thus, the light emitting elements 6 are located at a certain distance below the transparent substrate 2 and light can be reflected towards the viewer from the edges of the glass substrate 2 at the tile edges. Which light is reflected depends on which color element 6 of the edge-closest pixel structure is closest to the edge of the tile. The conventional color elements 6R, 6G, 6B of one pixel structure of an OLED are shown in FIG. 2 deposited onto a substrate 2—the electrodes and other layers are not shown. The color elements 6R, 6G, 6B are arranged in parallel, rectangular strips. The tile edge effect of such an arrangement differs depending upon which edge 10, 12 of the tile is closest to the pixel structure. If the edge (10) is parallel to the strips, then the light reflected towards the viewer from the edge of the glass substrate 2 at certain viewing angles will be dominated by the color of the closest color emitting element 6, e.g. 6B emitting blue light. This color will generate a color shift near the tile edge 10 generating a colored “halo” effect along edge 10, particularly at certain viewing angles. At the edge 12, all three color elements are equally close to the edge and therefore balanced colored light is reflected from the tile edge. This will not cause a color shift but may make the edge look slightly brighter or darker at certain viewing angles.
A further problem is the effect of viewing angle and parallax resulting in vignetting, e.g. with masks or other optical elements. As there is a significant distance in the viewing direction between the emitting point of light of the OLED pixel structure and the mask, a viewer will see light emitted from more or less of the pixel structure surface area depending on the angle of view (see FIG. 3a). This distance is determined as a minimum by the thickness of the substrate 2. This problem can be reduced by using non-occluded masks. A non-occluded mask has openings which are larger than the light emitting surface area of a pixel structure so that when viewed along a normal to the pixel structure, light from the complete emitting surface of the pixel structure can be seen (non-occluded mask 9 of FIG. 3a). However, even with a non-occluded mask 9, parallax effects caused by the mask can occur at larger angles of view, an effect called vignetting. The effect of vignetting is to reduce the light intensity from a part of the pixel structure. If this part has a predominance in one of the colors emitted from the pixel structure, the result will be a color shift which depends on viewing angle. In addition, mask misalignment can reduce the viewing angle in the direction in which an edge of the mask opening is closest to the pixel structure (see FIG. 3b) and/or can produce color shifts.
A further problem can occur with displays in which an array of optical elements is used, e.g. commercial large size displays such as e.g. as used in shopping malls, train stations, airports etc. Such displays may be arranged on a wall at a certain distance from the average viewer. Due to physical restrictions such as the height of the target observer who will normally be between 1 and 2 meters, it is not necessary for the display to emit light in a wide angle. Power and cost can be saved by directing the display beam only within a useful angle. One way of doing this is to provide an optical element such as a lens in front of the display that concentrates the emitted light within the desired viewing angle. For instance, each pixel structure may be associated with a lens. However, misalignments between the optical axis of the lens and the pixel structure as well as the difference in optical centers of gravity of each color element of a conventional pixel structure means that the color displayed depends on the angle of view. A known partial solution to this problem is described in US 2002/0050958 that requires each color element of a pixel structure to have its own lens. However, for high resolution displays this involves manufacturing and accurately placing very small lenses. The array of optical elements can have other functions besides the tailoring of the light distribution produced by the pixels.